In the field of orthopedic surgery, polymethyl methacrylate (PMMA) is widely used as a cement to anchor artificial joints to bone. The PMMA fills the free spaces between the prosthesis and the bone. A prosthesis fixed with bone cement offers very high primary stability combined with fast remobilization of patients. This is because the cemented-in prosthesis can be fully loaded very soon after the operation, since PMMA achieves the majority of its strength within the first 24 hours. Additionally, bone cement formulations can be modified to include active substances such as antibiotics. The antibiotics are locally released and act against bacteria precisely at the site where they are needed.
PMMA is supplied as a powder with liquid methyl methacrylate (MMA). When these two are combined, a dough is formed which can be used as a grouting agent to affix implants and remodel lost bone. While sticky, it does not bond to either the bone or the implant. Instead, it fills the spaces between the prosthesis and the bone preventing motion.
A disadvantage of bone cement is that the polymerization reaction is highly exothermic, and the bone cement can heat up to 82.5° C. while setting. This can cause thermal necrosis of neighboring tissue. Additionally, PMMA has been shown to cause stress shielding. PMMA has a Young's modulus between 1.8 and 3.1 GPa, which is lower than that of natural cortical bone. This causes the stresses to be loaded into the cement and thus the bone no longer receives the mechanical stimuli to continue bone remodeling. This can cause bone resorption to occur.
Mechanically, PMMA, when fully hardened, is strong but brittle. It is very strong in compression and resists creep; however, it has low ductility and a high susceptibility to stress crazing. This leads to larger cracks, relative motion between the implant and the bone, and eventual failure requiring the implant and PMMA to be removed and replaced. Thus there exists a clinical need for PMMA with improved fatigue resistance and enhanced mechanical properties.
In the field of orthopedics, hydroxyapatite, calcium phosphate, and tricalcium phosphate are widely used as a fillers to replace bone or as a coating to promote bone ingrowth. The material is remodeled by bone cells leading to healing. These biomaterials are, however, not load bearing. Screws, plates, and other hardware are used to transmit the load and the hydroxyapatite, calcium phosphate, and tricalcium phosphate is used as a space filler.
It has been found that these materials suffer from low crack resistance and low fatigue durability. This precludes them from being used as load bearing materials. Thus there is a clinical need for enhancing the fatigue properties and mechanical properties of these biomaterials in order to use them as structural biomaterials.